Fabrication of Novel III-N and III-V Modulator Structures by ECR Plasma Etching
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"(')Universityof Florida, Gainesville FL 32611
(2)Sandia National Laboratories, Albuquerque NM 87185 3
( )AT&T Bell Laboratories, Murray Hill NJ 07974 (4)US Army Research Laboratories, RTP, NC 27709 ABSTRACT
Quantum well microdisk laser structures have been fabricated in the GaN/InGaN, GaAs/AlGaAs and GaAs/InGaP systems using a combination of ECR dry etching (C12/CH 4 /H2 /Ar, BC13/Ar or CH 4/H2/Ar plasma chemistries respectively) and subsequent wet chemical etching of a buffer layer underlying the quantum wells. While wet etchants such as HF/H 20 and HCI/HNO 3/H20 are employed for AIGaAs and InGaP, respectively, a new KOHbased solution has been developed for AIN which is completely selective over both GaN and InGaN. Typical mask materials include PR or SiNx, while the high surface recombination velocity of exposed AIGaAs (- 105cm~sec-1) requires encapsulation with ECR-CVD SiNx to stabilize the optical properties of the modulators. INTRODUCTION Whispering gallery-mode microdisk lasers have optical modes strongly confined in a direction perpendicular to the wells comprising the disk ). Various methods have been
reported for directional coupling of light output from these lasers, including patterned asymmetries in the shape of the microdisk resonators (), and use of a double-disk structure.(3) A variety of different materials systems have been utilized to realize these low-threshold devices, including InGaAs/InGaAsP, 3) InGaAs/InGaP, GaAs/AlGaAs(4) and ZnSe/ZnCdSe.(5) There is interest in the use of GaN and related alloys for blue/UV emitters and detectors, sparked mainly by the development of highly luminescent light-emitting diode.(6) To date, the requisite dry and wet etching processes or indeed the growth of the necessary InGaN quantum wells have not been available to produce a microdisk structure in the nitrides. In this paper we report growth of a GaN-InGaN multiple quantum well layer structure by Metal Organic Molecular Beam Epitaxy (MOMBE), and a controlled two stage etching process for fabrication of a micro-resonator. EXPERIMENTAL The processing of the microdisck lasers is conceptually quite simple and is illustrated in Figure 1 for an optically pumped device. A photoresist, dielectric or metal mask is used during dry etching to form a 1.311m high cylinder. A selective wet chemical etch (selective for either InP, InGaP, or AIN over InGaAsP, GaAs, or GaN respectively, depending on the 115 Mat. Res. Soc. Symp. Proc. Vol. 405 01996 Materials Research Society
materials system being employed) is then used to undercut the material under the quantum wells, leaving a narrow support post which minimizes interference with generation of the whispering gallery modes around the top disk (see Figure 1 (c)). For the GaN/InGaN system we need to perform a wet etch undercut to form the disk, but the normal substrate used for growth (A12 0 3) is extremely difficult to etch. To circumvent this, we grew the InGaN/GaN quantum wells on a AIN buffer layer -0.5[tm thick. We then need to develop a selective wet etch for AIN ov
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